Cathode structures



Aug. 21, 1956 M. EBER ET AL 2,760,097

cATHc-DE STRUCTURES Filed nec. 22, 195o JZ j 31 il 55 i l x l 3:/ H 3J 4 Z5 Z5 e` 'g 29 l M Y 44% a1 1 M M P15 l i jig .Z1

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ATTORNEY CATHODE STRUCTURES Mortimer Eber, East Orange, land Walter R. Hayter, Jr., Little Falls, N. J., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania l Application December 22, 1950, Serial No. 202,246

6 Claims. (Cl. S13-'71) This invention `relates to cathodes for electronic tubes and more particularly to indirectly heated cathodes for magnetrons and other apparatus.

The newer type high temperature molybdenum or tungsten cathodes are coated with emissive material such as thoria and are indirectly heated by a conventional tungsten internal heater to temperatures of the order .of 2000 C. To obtain a heater insulator of suitable low vapor pressure, high strength, and low conductivity at this operating temperature, beryllium .oxide is used. However, there are two objectionable features to the use of this material.

` The first is the toxic eifect of beryllium compounds on the operators who handle it during manufacture. This effect alone has led many manufacturers to discontinue working with .this material and has limited the .source of supply to a single manufacturer. The second is the dilliculty of obtaining beryllium oxide of the required purity. Even the smallest amount of impurities will contaminate the emissive coating at the elevated operating temperatures employed. Further, conventional magnetron cathodes of this type require a pulse transformer in the cathode circuit which is large enough to carry not only the magnetron anode current, but also the cathode heating current.

Therefore it becomes desirable to use a cathode heating system which does not require an insulator. This is accomplished by heating the cathode structure by a high energy electron beam. The prior art indicates that cathodes of this type are solid bar stock having a hollow conical interceptor surface machined in one end. Impingement of the electrons from the electron beam upon this interceptor surface generates heat which is transferred to the cathode emitting surface by conduction. Because such cathode structures are either tantalum or molybdenum, their cost is extremely high .and their relatively large weight requires additional support With-in an electron discharge device.

Hence it has been found advantageous according to our invention to `provide a cathode lcomprising a hollow cylinder of molybdenum or tungsten which either carries or itself constitutes an temissive surface and inter-ceptors, such as discs of like metal to the cylinder, secured peripherally as by arc welding or brazing to the inside surface of the cylinder. Tapered holes coaxial with the cylinder and opening axially outwardly toward the electron beam source, are cut in each interceptor disc, said holes are made progressively smaller in each successive disc located in a direction away from said elect-ron beam. Collision of electrons from the beam with the interceptor discs generates heat which is transferred to the cathode cylinder .by conduction and radiation. By appropriate selection, construction and arrangement of parts, such as .by adjusting the position, thickness and number of electron beam interceptor discs, any desired temperature distribution along the cathode may be obtained. The cathode provided heats rapidly and has a low thermal inert-ia. i

In its general aspect, the present invention has the kobjective of overcoming the aforementioned disadvantages nited States Patent O 2,760,097 Patented Aug. 21, 1956 rice of .the prior art indirectly heated high temperature cath:

odes. i

, Speciiically, an object of the present invention is the elimination of the heater and the objectionable heater iusulator as a high temperature .cathode heating mea-us.`

Another and specitc object .of the present invention is the elimination of the costly and heavy bar stock cathodes heated by an electron beam. t

An additional object of the present invention is an 'indirectly heated cathode structure light in weight and easily supported within an `electron discharge device.

Another object of the present invention is `a cathode structure heatedby an electron beam and fabricated simply and cheaply.

Another object of the present invention is to provide a rapid heating cathode of `low thermal inertia.

An additional object of this invention apparent to those skilled in the art of magnetron usage is vthat a small pulse transformer capable `of carrying only the necessary D. C. current may .be `used in the cathode circuit.

Again, a further object of the present invention is a cathode structure heated by an electron beam and capable of any desired specific temperature distribution along its emissive surface.

A .still further obiect of the present invention is a cathode structure heated by an electron beam and capable of uniform temperature distribution.

Other objects of :the invention will appear to thosev skilled in the art to which it appertains as the description thereof proceeds, both by direct recitation thereof .and by implication from context.

Referring to the accompanying drawing in which like numerals of reference indicate similar parts throughout the several views:

Fig. l is asection of a magnetron and showing cathode structure and electron gun',`

`Fig. 2 is afsection along line Il-II of Fig. l;

Fig. 3 is a section of the cathode structure;

Fig. 4 is a top elevation of the `cathode structure; and

Fig. .5 is a detail section of `the cathode structure and showing an alternative embodiment of the interceptor discs.

Reierring'now to the drawing in detail, the magnetron 10, shown in Fig. l, comprises a magnetron body 11. of generally cylindrical conguration and provided therein with symmetrical end spaces 12, sailed vacuum-tight by cover plates 13. While a magnetron has been `arbitrarily selected for `illustrative purposes, the invention is not to be understood as restricted to employment with magnetrons only. Within the magnetron the body 11 provides `an anode 14 of generally cylindrical shape and shorter than the outside of the body so as to provide 4the said end spaces 12 `at opposite ends of the anode. Within the lanode and in direct communication with and between the end spaces is a cathode cavity 15 from which radiate a plurality ofsuitable cavities 1-6, here shown as cylindrical, functioning as cavity resonators, which are open-ended -into said end spaces 12. Each cavity resonator has, in the form shown, :a cylindrical portion having a longitudinal constricted capacitative slot 17 constituting a radial lateral opening from :the cylindrical portion .of the cavityinto `the hollow cathode cavity 15. :Said slots 17 extend the continues outwardly as it makes connection with the coaxial line output structure 19` which includes, as will be understood, an appropriate seal, not shown, so the evacuated condition of the interior of the magnetron body 11 will not be lost.

An indirectly heated cathode structure 2G passes axially through the cathode cavity 15 suitably equi-spaced laterally from the anode 14 and supported at its lower end from and joined thereto as by brazing to metallic lead-in and support rod 21. In turn said rod 21 extends through lower end space 12 and a coaxial outer tube 22 to which it is suitably vacuum sealed, as at 22a, and from which said rod has rigid support. Said outer tube 22 is connected as by brazing to the bottom cover plate 13 along the side wallsv of anappropriate center hole in said plate.

Cathode structure 20, as shown in Fig. 3, comprises a hollow metallic cylinder 23, suitably molybdenum or tungsten, which either carries or itself constitutes an emissive surface and also comprises a plurality of transverse, parallel, longitudinally equi-spaced, metallic interceptors, such as discs of like metal, to the cylinder, joined peripherally, as by arc welding, or brazing, to the inner surface of said cylinder 23. In the present showing of Fig. 3, the several interceptors, or specifically, discs 24, are of like thickness one to the other as a factor to achieve, in this case, uniform temperature distribution along the length of the cathode. Each disc except the disc nearest the supported end of the cathode contains a frusto-conical center hole 25, tapering outwardly from the cathode axis in a direction toward the open end of said cathode. Said disc nearest the supported end of the cathode is joined axially, as by brazing, to lead-in and support rod 21.

A further factor for obtaining uniform temperature distribution along the cathode surface is the taper of each Ihole 25. By virtue of said taper, the side wall of said .hole 25 presents a transverse component of area for intercepting of the beam, and the tapers of successive holes vary in such manner and amount to make the beamintercepting areas of the several holes equal to each other. In a direction away from the open end of the cathode the holes 25 in successive discs 24 are made progressively smaller such that the smaller diameter of a given disc is .approximately equal to the larger diameter of the next successive disc. The smaller diameter. of the frustoconical hole in the disc 24 next to the last from the supported cathode end is appropriately selected so that the impingement area on the last disc is approximately equal to the side wall transverse component impingement area `of each of the frusto-conical holes 25 in the other inter- Vceptor discs. However, it will be understood that any desired specific temperature distribution along the cathode vsurface can be obtained by adjusting the distance between interceptor discs and by altering the thickness and the -of said anges being shown depending in the same general direction from the plate. Said flange 27 at the outer periphery is conveniently arc Welded or otherwise secured `to the inside surface of the cylinder 23 so the plates are .both heat conductive thereto and rigidly held perpendicular to the axis of said cylinder. The inner flange 28 constitutes, in this modification, the frusto-conical beam inter- 'cepting area similar to the beam-intercepting areas of holes 25 above described.

By increasing the length (as shown) or the taper of said surface 28, a greater transverse component of interceptor area is presented to an electron beam and more heat is consequently absorbed by the interceptor 26. This form of interceptor reduces appreciably the weight of cathode structure20.

Joined to upper cover plate 13 along the side walls of a suitable center hole, as by brazing, and projecting outwardly from said cover plate coaxially with the cathode structure 2i) and magnetron body 11 is metallic drift tub: 29. On the outer end of and coaxial to drift tube 29 is mounted a suitable electron gun 30. Said gun 30 comprises, in general, an appropriate oxide coated cathode emitting surface 3l, indirectly heated by a tungsten coil heater 32, a cathode electrode 33 and an accelerating anode 34, having center aperture 37, in cooperative relation with the cathode for concentrating the electrons emanating from the cathode surface 31 into a stream of restricted cross section coaxially down the drift tube 29 and against the interceptor discs 24, of cathode structure 2t). Cathode surface 31, heater 32, and cathode electrode 33 are connected together electrically at the same potential. The accelerating anode 34, the magnetron body 11, and cathode structure 2i) are all at ground potential. The electron gun cathode structure is insulated from and joined to the accelerating anode housing by the usual vacuum-tight metal to glass seal.

Suitably situated symmetrically about the drift tube 2) and the cathode tube 22 and here shown as having the spaced relation to the magnetron body 11 is an appropri ate electromagnet 35. As shown, the pole pieces 36 of the magnet are grooved appropriately along the vertical centerline thereof to t with the sufficient clearance into position around drift tube 29 and cathode tube 22.

According to our invention, after heater 32 has preheated the cathode surface 31 of the electron gun to emitting temperature, an appropriate negative direct current voltage of approximately l() kv. is applied to the cathode electrode 33. The approximate l0 kv. drop between the cathode electrode and the grounded accelerating anode 33 concentrates a limited cross section beam of high velocity electrons through the accelerating anode aperture 37 and down drift tube 29 against interceptor discs 24 of cathode structure 20. The cross section of electron flow is suitably limited, in the uniform temperature distribution case, to the circular area having the large diameter of the frusto-co-nical hole in the leading interceptor disc. Variation in said cross section of the electron stream may be obtained by combined changes in the size of aperture 37 of the accelerating anode 34, the distance between cathode electrode 33 and accelerating anode 34, the size of the cathode emitting surface 31, and the shape of the cathode electrode and accelerating anode. The number of electrons and the energy per electron in the stream may be increased or decreased by raising or lowering the negative direct current voltage on cathode electrode 33.

When the electron beam impinges upon the side wall areas of the series of progressively smaller frnsto-conicsl holes 25, equal horizontal cross sections of the beam are intercepted by the side wall of each hole. As the high velocity electrons bombard the impingement area of the interceptor, they impart their kinetic energy of motion to the interceptors in the form of heat which in turn is transferred by conduction and radiation to the cathode cylinder 23. As mentioned above, in the uniform temperature case, the preferred embodiment of the cathode structure 20 comprises longitudinally equi-spaced parallel interceptors 24 of uniform thickness joined along their peripheries to the inner surface of cathode cylinder 23. This uniformity of the interceptors both as to position with respect to the cathode cylinder and physical thickness in addition to uniform energy interception by the side walls of the frusto-conical holes 25 in each interceptor insures uniform heat distribution along the cathode cylinder 23, However, if, for example, a heat concentration were desired in the center of the cathode cylinder 23, an increase in the number of interceptors 24 in this area, closer spacing of the existing interceptors, increasing the thickness of the existing interceptors, or increasing the impingement area of the frusto-conical holes 25 in interceptor-s in the selected area to intercept a larger portion of the electron beam; any or a combination of the above would achieve the desired result.

Further, according to our invention, the alternative embodiment of cathode structure 20, as shown in Fig. 5, may be employed particularly where a lighter cathode structure and a greater impingement area is required. Here the frusto-conical center interceptor surface may be spun inwardly to any desired length of taper without the added weight entailed in the use of a thicker interceptor disc 24 of the preferred embodiment.

Thus, it will be seen from the foregoing description that the present invention has overcome the defects and difficulties of the prior art indirectly heated high temperature cathodes used in electron discharge devices. Specifically, the present invention has eliminated the heater and objectionable beryllium oxide insulator as a cathode heating means. The cathode structure 20 of the present invention is light in weight and easily supported within an electron discharge device. It further eliminates the expensive and heavy conically recessed bar stock cathodes of the prior art also heated by an electron beam. Again, the cathode structure of our invention is simply and easily manufactured. Cathode structure 20 can be heated uniformly along its entire surface by the electron beam from gun 30, mounted coaxially with structure 20 on drift tube 29, provided equi-spaced interceptors 24 of like thickness containing equivalent interceptor areas in the frusto-conical holes 25 are employed. However, by varying the spacing, the number of interceptors employed, the thickness of the discs or the impingement area of each hole any desired specific temperature distribution along the cathode emissive surface can be obtained.

Although preferred embodiments of our invention have been described, it will be understood that modifications may be made within the spirit and scope of the appended claims.

We claim:

1. An electron discharge device comprising an anode, a cathode supported at one end within said device, said cathode having an emissive coated hollow body and having in said body interceptors joined at their peripheries to the inner surface at points directly underlying said emissive coating of said body to provide a short path of heat conduction to said emissive coated body, said interceptors being provided with center holes, said holes being progressively smaller in each successive interceptor in a direction away from the unsupported end of said cathode, and means for projecting a stream of electrons axially into said cathode body against the side walls of said holes of said interceptors, thereby heating said interceptors Within the cathode body and in turn heating said cathode body by conduction along and radiation from said interceptors.

2. An electron discharge device comprising an anode, a cathode supported at one end within said device, said cathode having an emissive coated hollow metallic cylinder and having in said cylinder parallel metallic interceptor discs joined at their peripheries to the inner surface of said cylinder at points directly underlying said emissive coating of said cylinder to provide a short path of heat conduction to said cylinder, said discs being provided with frusto-conical center holes coaxial with said cylinder and tapering axially outwardly toward the unsupported end of said cathode, said holes being progressively smaller in each successive disc in a direction away from said unsupported end of said cathode, and means for projecting a stream of electrons axially into said cathode cylinder `against the side walls of said holes of said interceptor discs, thereby heating said discs within the cathode cylinder and in turn heating said cathode cylinder by conduction along and radiation from said interceptor discs.

3. An electron discharge device comprising an anode, a cathode supported at one end within said device, said cathode having an emissive coated hollow body and having in said body parallel interceptors, said interceptors having a peripheral ange for joining to the inner surface of said body at points directly underlying said emissive coating of said body, to provide a short path of heat conduction to said emissive coated body, and center holes formed by flared-in frusto-conical flanges, coaxial with said cylinder and tapering outwardly toward the unsupported end of said cathode, said flanges being progressively smaller in each successive interceptor in a direction away from said unsupported end of said cathode, and means for projecting a stream of electrons axially into said cathode body, against the side walls of said frusto-conical flanges of said interceptors thereby heating said interceptors Within the cathode body and in turn heating said cathode body by conduction along and radiation from said interceptors.

4. A cathode for an electron discharge device having a closed end and an open end and having a hollow cylindrical body, the Outer surface of said hollow cylindrical body being emissively coated, said body having parallel metallic interceptors joined at their peripheries to the inner surface of said hollow cylindrical body at points ydirectly underlying said emissive coating of said body to provide a short heat conduction path to said emissively coated body, said interceptors being provided with frusto-conical center holes, coaxial with said cylindrical body and tapering axially outwardly toward said open end of said cathode, said holes being progressively smaller in each successive interceptor in a direction away from said open end of said cathode body, and means for projecting a stream of electrons axially into said hollow cylindrical body against the side walls of said center holes of said interceptors, said stream of electrons entering said cathode at said open end.

5. A cathode for an electron discharge device having a closed end and an open end and having an emissive coated hollow metallic cylinder, said cylinder having parallel metallic interceptor discs joined at their periphcries to the inner surface of said cylinder at points directly underlying said emissive coating of said cylinder to provide a short path of heat conduction to said cylinder, said discs being provided with frusto-conical center holes coaxial with said cylinder and tapering axially outwardly toward said open end of said cathode, said holes being progressively smaller in each successive disc in a direction away from said open end of said cathode cylinder, and means for projecting a stream of electrons axially into said metallic cylinder against the side walls of said center holes of said interceptor discs, said stream of electrons entering said cathode at said open end.

6. A cathode for an electron discharge device, having a closed end and an open end and having an emissive coated hollow cylinder, said cylinder having parallel interceptors, said interceptors having a peripheral ange for joining to the inner surface of said cylinder at points directly underlying said emissive coating to provide a short path of heat conduction to said cylinder and aredin frusto-conical flanges tapering outwardly toward said open end of said cathode and being progressively smaller in each successive interceptor in a Edirection away from said open end of said cathode cylinder, and means for projecting a stream of electrons axially into said` hollow cylinder against the side walls of said frusto-conical flanges of said interceptors, said stream of electrons entering said cathode at said open end.

References Cited in the tile of this patent UNITED STATES PATENTS 2,160,798 Teal May 30, 1939 2,250,511 Varian et al. July 29, 1941 2,284,405 McArthur May 26, 1942 2,450,763 McNall Oct. 5, 1948 2,567,624 Thomson et al. Sept. 1l, 1951 

